1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device and, especially, to a structure of electrode layers of a SAW filter.
2. Description of the Related Art
A conventional SAW device is described in, for example, Japanese Patent Application Kokai No. 9-69748. FIGS. 7(a)-(c) show such a conventional SAW device.
In a band filter having a center frequency of 1.5 GHz, an electrode wire is required to have a width of 0.7 xcexcm. When a large electric power is applied to the SAW device having such a fine electrode, the strain produced by the SAW causes a stress in the electrode layer. When this stress exceeds the critical limit stress of the electrode layer, the atoms of the electrode material, or aluminum (Al), move along the grain boundary. As a result, hillocks and voids are formed and the electrode is broken so that the characteristics of the SAW device deteriorate. To solve the problem, an aluminum alloy layer containing copper (Cu) has been used as the electrode material since the aluminum alloy layer withstands a larger electric power than the pure aluminum layer. Also, it has been attempted to reinforce the electrode layer by adding titanium (Ti), nickel (Ni), or palladium (Pd).
The device according to the prior art, however, does not achieve satisfactory electric power durability and low insertion loss for the electric power necessary for the transmission stage of a mobile telephone. For example, an analog cellular telephone is required to withstand application of an electric power of 1 W or more, and the insertion loss is as low as that of the common dielectric filter. However, it failed to achieve them.
In order to withstand a large electric power, the rate of an added metal may be increased. However, the increased metal causes an undesirable increase in the specific resistance of the alloy layer and the insertion loss.
Accordingly, it has been proposed to provide an SAW device comprising an inter-digital transducer (IDT) electrode capable of withstanding a large electric power application and preventing increase in the insertion loss.
According to the prior art, the IDT electrode provided on the surface of a piezoelectric substrate is formed by the alternate lamination of an Al layer and a layer made of a conductive material having an elastic constant greater than that of the Al layer. In addition, the number of the laminated layers of the conductive material and Al is two or more.
The thickness of each Al layer is 150 nm or less. The thickness of each layer of the conductive material having an elastic constant greater than that of the Al layer is smaller than that of the Al layer. As a result, an increase in electric resistance of the electrode is prevented, the mechanical strength of the electrode is increased, the production of the hillocks and voids is prevented, and the electrode withstands a larger electric power than before.
In addition, the outermost layer of the laminated electrode of the prior art is made of either the Al layer having a thickness of 50 nm or less or the layer of a conductive material having an elastic constant greater than that of the Al layer, so that the device prevents production of the hillocks and voids formed in the outermost layer and withstands a larger electric power than before.
The production mechanism of the hillocks and voids will be described below.
When the SAW device is excited, a strain is generated in a piezoelectric substrate, causing a stress applied to the IDT electrode. When the stress exceeds the limit of the layer, Al atoms in the electrode propagate through the grain boundary and move to the surface of the electrode to form the hillocks. When Al atoms move into the surface, the voids of the Al atoms are produced in the layer. When many hillocks and voids are produced, the electrode layer is broken, and fluctuation of the frequency and increase in the insertion loss occur, and finally the SAW device is unable to function properly.
Such production of hillocks and voids due to the movement of Al atoms decreases as the mechanical strength of the layer increases or the particle size of the Al layer decreases. Also, it is known that the movement of Al atoms is suppressed when atoms of Cu and Ti precipitate into the grain boundary. Accordingly, attempts have been made to add various materials. However, the amount of the addition was limited because the increased concentration of the added material increases the specific resistance of the layer, leading to a significant increase in the insertion loss.
According to the prior art, even if the concentration of the added material is increased in a single alloy layer, the grain boundary exists continuously from the interface of the piezoelectric substrate and electrode to the surface of the electrode. When stress applied to the electrode exceeds the limit of the alloy electrode, the Al atoms move up to the electrode surface through the grain boundary, thereby producing hillocks.
On the basis of such recognition, the prior art prevents not only the particle size of Al layer from increasing but also the Al atoms from moving up to the electrode surface through the grain boundary, by dividing the Al layer into a plurality of layers with a material having an elastic constant greater than that of the Al layer.
Furthermore, by providing a layer of material having an elastic constant larger than that of the Al layer between the Al layers, the elastic strength of the entire electrode layer is enhanced so that the electrode withstands a large stress. Incidentally, when the thickness of each of the laminated Al layers is large, the Al atoms move in the lateral direction due to the stress caused by excitation, and side hillocks are formed. The side hillocks cause not only deterioration of the electrode but also a short-circuit by contacting with the adjacent IDT electrodes.
A solution to the above problems is to reduce the thickness of the laminated Al layer. Experiments show that the thickness of the Al layer no more than 150 nm, preferably no more than 100 nm is sufficient to withstand electric power application of 1 W or more for use in the transmitter stage. Also, since a conductive material having an elastic constant greater than that of an Al layer generally has a density higher than that of the Al layer, it is preferred to form a conductive layer as thin as possible, at least thinner than the Al layer, which prevents increase of the electric resistance of the electrode.
In summary, the SAW device according to the prior art comprises a piezoelectric substrate and a comb-shaped IDT electrode provided on the surface of the piezoelectric substrate. The IDT electrode is composed of the first layer made of only Al and the second layer made of only a conductive material having an elastic constant and hardness greater than those of the first layer.
The IDT electrode comprises a plurality of the first layers and a plurality of the second layers. The thickness of the first layer is 150 nm or less and the thickness of the second layer is smaller than that of the first layer. That is, the barrier metal is made thin because of its large resistance. Thus, the second layer is made thin so that the device is not influenced by high-frequency waves.
In FIGS. 7(a)-7(c), the electrodes have different numbers of laminates. The sectional views illustrate only one IDT electrode. In the drawings, reference numbers 31 to 33, 35 to 38, and 40 to 45 are Al layers, and 51 to 52, 53 to 55, and 56 to 60 are conductive material layers having an elastic constant greater than that of the Al layers. As shown in FIG. 7(a), the IDT electrode 2 comprises the layers 31-33 and 51-52. The IDT electrode in FIG. 7(b) comprises layers 35-38 and 53-55. The IDT electrode in FIG. 7(c) comprises layers 40-45 and 56-60. The IDT electrode 2 is provided on a piezoelectric substrate of lithium tantalate 1.
It is noted that when an Al-1 Wt % Ti alloy layer or an Al-1 Wt % Cu alloy layer is employed in the electrode, hillocks are produced.
The prior art describes that if the electrode is composed by alternately laminating a layer made of only Al and a layer made of a conductive material having an elastic constant greater than that of the Al layer, the withstanding characteristic to electric power increases. However, nothing is described in the prior art about a naturally oxidized layer or an insulative layer. It is discovered that employing these layers in the electrode is effective in solving the above problems.
Accordingly, it is an object of the present invention to provide a SAW device using the naturally oxidized layer or insulative layer to improve the power withstanding characteristics and prevent the production of hillocks and voids.
According to the present invention, the following devices are provided.
(1) A surface acoustic wave (SAW) device comprises a piezoelectric substrate, at least one alloy layer made of aluminum (Al) and copper (Cu), at least one oxide layer, and at least one metal layer made of a metal other than Al, wherein the alloy layer, oxide layer, and metal layers are laminated one upon another.
(2) A surface acoustic wave (SAW) device according to the above para. (1), which further comprises a ground layer made of Chrome (Cr) or Titanium (Ti) and provided on the piezoelectric substrate.
(3) A surface acoustic wave (SAW) device according to the above para. (1), wherein the alloy layer, oxide layer, and metal layer are provided in this order from the piezoelectric substrate.
(4) A surface acoustic wave (SAW) device according to the above para. (1), wherein the alloy layer and metal layer are provided in this order from the piezoelectric substrate, and the oxide layer is interposed in the alloy layer.
(5) A surface acoustic wave (SAW) device according to the above para. (1), wherein the alloy layer, metal layer, and oxide layer are provided in this order from the piezoelectric substrate.
(6) A surface acoustic wave (SAW) device according to the above para. (1), wherein the alloy layer and oxide layer are provided in this order from the piezoelectric substrate, and the metal layer is provided as an uppermost layer.
(7) A surface acoustic wave (SAW) device comprises a piezoelectric substrate, at least one first metal layer made of a metal other than aluminum (Al) and copper (Cu), at least one second metal layer made of a metal other than Al, Cu, and the metal used in the first metal layer, and at least one alloy layer made of Al and Cu, wherein the first metal layer, second metal layer, and alloy layer are provided in this order from the piezoelectric substrate.
(8) A surface acoustic wave (SAW) device comprises a piezoelectric substrate, at least one first metal layer made of a metal other than aluminum (Al) and copper (Cu), at least one oxide layer, at least one second metal layer made of a metal other than Al, Cu, and the metal used in the first metal layer, and at least one alloy layer made of Al and Cu, wherein the first metal layer, oxide layer, second metal layer, and alloy layer are provided in this order from the piezoelectric substrate.
(9) A surface acoustic wave (SAW) device according to one of the above para. (1)-(6) and (8), wherein the oxide layer is a naturally oxidized layer.
(10) A surface acoustic wave (SAW) device according to one of the above para. (1)-(2), wherein the alloy layer is made by sputtering.
(11) A surface acoustic wave (SAW) device according to one of the above para. (1)-(2), wherein the metal layer is made of titanium (Ti).
(12) A surface acoustic wave (SAW) device according to one of the above para. (7)-(8), wherein the first metal layer is made of chrome (Cr) and the second metal layer is made of titanium (Ti).